I'm sure there is lots of useful info out there but I recently watched this short film on Korduroy tv on fin design that I found very informative. The film discusses the notion that changing your fin setup can significantly change the way your board performs e.g. a fin that is more vertical with more surface area than one that is swept-back will pivot more than the swept-back fin which push the board down the line.

A thruster (3-fin setup) will anchor the tail down so that you feel the drive more than other fin setups. It also allows the surfer to work the fins from side-to-side to generate speed ('speed-pumps'). Quad fin will allow the surfer to get the board on the rail very easily whereas a five-fin setup can enhance the drive of the board. The example in the film shows a five-fin setup where the fifth fin (the one in the middle close to the tail) is a keel fin i.e. a very small one.

The following quote from Tom Curren in issue 92 of The Surfer's Path gives a good overview of twin fins vs single fins.

"...the performances (of twin fins) ... there was no comparison with single fins. For more performance surfing, twin fins were definitely a lot faster. The problem is that they just didn't work as good backside, and that was always the problem."

For some great examples of single fin surfing check out any videos you can find of Terry Fitzgerald or more recently his sons Joel and Kye here.